The present invention relates to a digital scan converter (DSC) and more particularly to a DSC embodying a means for predefining a relationship between each picture element whose value is to be calculated to produce a converted image, to two or more picture elements in the original image. By predefining the function that relates the picture elements of the original and final images and by employing a novel interpolation circuit, the picture elements of the converted image can be calculated in real time.
In general, digital scan converters are used for converting image data from one display format to another. For example, ultrasound probes generates the familiar rotating beams which are used for scanning a patient or for that matter any object whose inner structure is to be studied, on radar systems generating PPI displays. Echoes from reflecting boundaries or inhomogenieties in the object can be used directly, and without further processing, for controlling the brightness level of a CRT to generate the familiar rotating polar vector display format.
On the other hand, in many applications it is useful, or even required, to display the echo information in accordance with a raster scan television display format rather than the sterile appearing and inflexible polar display format referred to above. Television monitors are more available and provide greater flexibility which can be used to design display formats which suit individual needs. The conversion of the echo information from its original form to a final form which is acceptable as an input to a raster scan display system is referred to as scan converting. When this function is carried out digitally, the apparatus is a digital scan converter.
DSC's are not new and have been available since the early 1970's. Known DSC's, however, have not to date been able to combine economically in one embodiment the most desirable features that users thereof require. These include the ability to calculate, rather than approximate, the value of each pixel in the final image by noting the relative positions of the original and final picture elements and applying an interpolation process to calculate the values of the final picture elements. Secondly, the calculations must be produced in real time, or as fast as the calculated pixels are displayed on the raster scan display monitor. Finally, to reduce costs the above objects must be attained without parallel processing. That is, without hardware duplication for processing multiple pixels simultaneously or the employment of very large buffer memories. Attainment of the above objectives has been difficult particularly because converting, as done most often, from polar formatted scan data to rectilinear image data formats of raster scan devices must overcome several stumbling blocks. Firstly, the number of pixels in the rectilinear scan format is significantly greater than the corresponding number of pixels in the polar format. Thus, the number of rectilinear pixels which must be related to the original pixels and whose values must be calculated based thereon is quite large.
Secondly, the order in which the polar pixels are received from the probe and the order in which they are displayed differ significantly. In the original image the pixels are sequentially located along each polar vector and follow their sequential scanning order. In marked contrast, the scan lines of a raster scan system generally intersect, approximately perpendicularly, the polar scan lines. This requires the storage of at least one complete image or frame of data in a memory. The DSC must access and retrieve the data not according to the order in which it was stored but according to the order in which the converted data is displayed. Furthermore, the same data may be used for calculating several reformatted image pixels. Ordinarily, this requires repeated retrieval of the same information from the memory.
Considering further that the pixel values of the rectilinear system must be produced in real time, i.e., at a rate of 30 frames per second and approximately 525 lines per frame, the number of memory accesses and associated calculations is too large to allow real time operation. Therefore, and as previously noted, prior art digital scan converting techniques involve compromises in the areas of real time operation, image resolution, or hardware economics.
The article Digital Scan Converters in Diagnostic Ultrasound Imaging by Jonathan Ophir et al, Proceedings of the IEEE, Vol. 67 No. 4 April '79, pages 654-664 presents a detailed discussion of DSC's. It summarized the techniques and construction of DSC's at the time of its publication. However, the unique approach to and method of digital scan converting according to the invention are not disclosed or suggested by this article.
U.S. Pat. No. 4,212,072 relates to a DSC with a programmable transfer function. This reference discloses a unique transfer function in which current pixel values are calculated based on previous pixel values to obtain more accurate results. However, preselection of pixel groups and predefinition of interpolation factors to enable real time operation is not discussed in this reference.
U.S. Pat. No. 4,215,414 discloses a new video output processing scheme for smoothing a digital display. According to this patent a pseudogaussian smoothing function is provided by relating the value of each pixel, not only to the pixels which lie horizontally and vertically adjacent to it, but which includes diagonally adjacent pixels also. Thus, this patent deals with an apparatus that relates pixels of the same image to one another. In contrast, the present invention deals with a DSC which calculates new pixel values based on pixel values from another imaging format. The present invention does not deal with a smoothing circuit for controlling pixel to pixel variations.